Semi-rigid polyurethane foams with a self-forming skin are generally called integral skin foams. Because of their flexibility and good touch, integral skin foams are extensively used in steering wheels and interior automotive trims such as arm rests and crash pads. With the recent trend for using larger windshields and windows in automobiles, greater light-stability is, more than even before, now required for interior automotive trims. This is all the more true because many different colors have come to be used in the trims and they are often pale in tone.
However, the semi-rigid integral foams made from aromatic isocyanates which are currently used in interior automotive trims yellow upon exposure to sunlight, particularly ultraviolet radiation, and depending on the color of the trims, the yellowing is so conspicuous that it reduces the aesthetic appeal of the trims. A plausible reason for this phenomenon is that the aromatic ring in the chemical structure of the foam changes to a quinoid as shown below: ##STR1##
One method which has been proposed for solving this problem is the in-mold coating process wherein a urethane elastomer made of an aliphatic isocyanate or alicyclic isocyanate is dissolved in a solvent; a coat of the solution is applied onto the inner wall of a mold with a spray gun; and a semi-rigid urethane is poured into the mold. However, because of the additional steps of applying and drying the mold coat, the in-mold coating process has a longer molding cycle. Furthermore, in order to provide a coat of a thickness of about 50 microns that is not necessary for ensuring sufficient light stability, repeated coatings must be conducted and this leads unavoidably to a prolonged time for coating and drying. As another disadvantage, it is difficult to form a uniform thickness of coat on the inner wall of a mold having a complicated shape.
The commonest way to prevent the photo-degradation of semi-rigid integral forms is by the addition of ultraviolet absorbers and antioxidants. This method is effective in preventing the occurrence of cracks in the foam but is not highly effective in preventing the discoloration of the foam. A method has been proposed for preventing both yellowing and cracking by using a urethane foam made from an aliphatic or alicyclic isocyanate wherein the NCO group of the isocyanate is not directly bonded to the aromatic group. According to Japanese Patent Publications Nos. 7956/78 and 3365/80, a non-yellowing urethane foam is prepared by using a mixture of aliphatic and aromatic polyisocyanates. However, the urethane foam incorporating the aromatic isocyanate cannot be completely protected from yellowing.
Japanese Patent Publication No. 15599/79 shows formulations wherein various aliphatic and alicyclic isocyanates are combined with a polyether polyol, a crosslinking agent, an organometallic catalyst and an antioxidant. The antioxidant is indispensable for preventing the melting of the urethane foam upon exposure to ultraviolet radiation, but the amount of the antioxidant incorporated in these formulations is about 3.4 wt% which is unusually greater than the conventional amount. Therefore, the antioxidant may bleed in the foam or otherwise deteriorate its properties.
Japanese Patent Publication No. 43167/82 describes a formulation wherein a hexafunctional polyether polyol and diethanolamine are used as crosslinking agents, together with isophorone diisocyanate, hydrogenated, 4,4'-diphenylmethane diisocyanate, an ultraviolet absorber and an antioxidant. However, because of the presence of the hexafunctional polyether polyol, this formulation is not expected to have good elongation. In addition, the amounts of the ultraviolet absorber and the antioxidant used therein are as high as about 3% by weight.
The problem common to these prior art techniques is that the melting of the foam prepared from aliphatic and alicyclic isocyanates due to ultraviolet radiation cannot be completely prevented unless large quantities of an ultraviolet absorber and an antioxidant are incorporated.